Since the foamed article comprising a thermoplastic resin has generally lightweight, good heat insulating property and cushioning property against outside stress, it is widely used for many fields such as heat-insulating material, cushioning material, core material and food containers.
In the foamed article comprising a polyolefin resin used as a base resin, low-density polyethylene is mainly utilized. But the polyethylene resin has a defect of insufficient heat resistance. On the other hand, a polypropylene resin has high elasticity and is excellent in chemical resistance, heat resistance, hinge characteristics and the like. Consequently, when the foamed article is prepared with the polypropylene resin, an extremely wide range of applications can be expected. But it is extremely difficult to prepare the foamed article having a large wall thickness as in case of a polypropylene resin, since viscosity and tension at melting is low, and strength of the cell wall at foaming is not sufficiently kept.
As an attempt to overcome these difficulties, foaming methods using polypropylene with a high melt tension are disclosed in Japanese Unexamined Patent Publication No. 43766/1978, Japanese Unexamined Patent Publication No. 197132/1982, Japanese Unexamined Patent Publication No. 363227/1992, and the like. However, even if these methods are used, satisfactory foamed boards have not yet been obtained.
In recent years, a method for producing sheet-like foamed article by using polypropylene with a long chain branching structure has been reported as in Japanese Unexamined Patent Publication No. 506875/1993. However, in the above-mentioned method, only the foamed article with wall thickness less than 10 mm can be prepared, and the foamed article having excellent impact resistance and cushioning property can not be obtained due to corrugates or voids generation.
Japanese unexamined Patent Publication No. 504471/1996 discloses a foamed article of propylene having an expansion factor F (density of foamed article.times.average cell particle size.times.tan .delta..sup.0.75) of less than 1.8. However, because of a special method of a convergent foam by a porous die, the method has a defect that thick foamed article can be obtained only by the foamed article with defects in quality.
Japanese Unexamined Patent Publication No. 252318/1995 discloses a polypropylene resin for foamed extrusion article having thick wall, of which biaxial elongation viscosity is specified. However, the resin is a special linear polypropylene resin containing a super high molecular weight component, which has Mz of more than 8.times.10.sup.6 and Mz/Mw of more than 10, and the disclosed preparing method is a special method of the two-step polymerization.
On the other hand, as the foamed article of a mixed resin comprising polyolefin and polystyrene, a foamed article of a mixture using a block copolymer of hydrogenated styrene and butadiene as a compatibilizer is disclosed in Japanese Unexamined Patent Publication No. 32263/1975. However, in this publication, only polyethylene is described as polyolefin and polypropylene is not described.
In addition, Japanese Unexamined Patent Publication No. 174237/1987 discloses a foamed article of mixed resin comprising polyolefin and polystyrene by using a special hydrogenated block copolymer of styrene and butadiene block copolymer, in which an amount of 1, 2-binding butadiene and styrene are specified. And in this publication, an example using polypropylene as polyolefin is also disclosed. But according to this method, only the foamed article having several mm thickness is obtained.
As described above, a method for preparing the foamed article having a low density, large thickness, a high closed cell ratio and a small average cell diameter according to extrusion foaming has not yet been found by improving foamability of a polypropylene resin.